1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a wide viewing angle in-plane switching mode liquid crystal display device.
2. Discussion of the Related Art
Twisted nematic liquid crystal display devices(hereinafter TN LCDs) having high image quality and low consumption electric power are widely applied to flat panel display devices. TN LCDs, however, have a narrow viewing angle due to refractive anisotropy of liquid crystal molecules. This is because prior to applying voltage, liquid crystal molecules are horizontally aligned relative to the substrate but become nearly vertically aligned relative to the substrate when voltage is applied to a liquid crystal panel.
Recently, in-plane switching mode liquid crystal display devices(hereinafter IPS-LCDs) have been widely studied in which viewing angle characteristic is improved and the liquid crystal molecules are nearly horizontally aligned.
FIG. 1A is a plan view of a unit pixel of a conventional IPS-LCD. As shown in the drawing, a unit pixel region is defined by a gate bus line 1 and a data bus line 2 in which the lines are arranged perpendicularly and/or horizontally in a matrix on a first substrate 10. A common line 3 is arranged parallel to the gate bus line 1 in the pixel region. A thin film transistor(TFT) is formed of a crossing area of the data bus line 2 and the gate bus line 1. As shown in FIG. 1B which is a sectional view taken along line I-I' of FIG. 1A, the TFT includes a gate electrode 5, a gate insulator 12, a semiconductor layer 15, a channel layer 16, and source/drain electrode 6. The gate electrode 5 is connected to the gate bus line 1, and the source/drain electrode 6 is connected to the data bus line 2. The gate insulator 12 is formed on the entire surface of the first substrate 10.
A common electrode 9 and a data electrode 8 are formed in the pixel region. The common electrode 9 is formed with the gate electrode 5 and connected to the common line 3. The data electrode 8 is formed with the source/drain electrode 6 and electrically connected to the source/drain electrode 6. Further, a passivation layer 20 and a first alignment layer 23a are deposited on the entire surface of the first substrate 10.
On a second substrate 11, a black matrix 28 is formed to prevent a light leakage which may be generated around TFT, the gate bus line 1, and the data bus line 2. A color filter layer 29, and a second alignment layer 23b are formed on the black matrix 28 in sequence. Also, a liquid crystal layer 30 is formed between the first and second substrates 10, 11. When voltage is not applied to LCD having the above structure, liquid crystal molecules in the liquid crystal layer 30 are aligned according to alignment directions of the first and second alignment layers 23a, 23b, but when voltage is applied between the common electrode 9 and the data electrode 8, the liquid crystal molecules are vertically aligned to extending directions of the common and data electrode. As in the foregoing, since liquid crystal molecules in the liquid crystal layer 30 are switched on the same plane at all times, a grey inversion is not created in the viewing angle directions of up and down direction, and right and left direction.
However, in the conventional LCD having the above structure, an aperature ratio is less than desired because the data electrode and the common electrode are opaque. Also, since a short by coupling the common electrode and said gate bus line in the manufacturing process of the LCD is often generated, the yield goes downed. Further, for the gate insulator and the passivation layer between the data electrode and the common electrode, a high driving voltage for switching liquid crystal molecules is required.